Fallback on prompt if env, stdin or auth file do not provide both username and password

[pdecat]

This PR allows to only pass the username in any of the supported configuration method.

For example, when using the environment variables method but only with username defined, it will prompt for the missing password:

# alias my-aws-adfs='username=patrick@mydomain.org aws-adfs login  --env --profile=myprofile --region=eu-west-1 --adfs-host=sso.mydomain.org --ssl-verification'
# my-aws-adfs
Failed to read password from env
Password: *****
Sending request for authentication
Waiting for additional authentication
Going for aws roles

        Prepared ADFS configuration as follows:
            * AWS CLI profile                   : 'myprofile'
            * AWS region                        : 'eu-west-1'
            * Output format                     : 'json'
            * SSL verification of ADFS Server   : 'ENABLED'
            * Selected role_arn                 : 'arn:aws:iam::0123456789:role/myrole'
            * ADFS Server                       : 'sso.mydomain.org'
            * ADFS Session Duration in seconds  : '28800'
            * Provider ID                       : 'urn:amazon:webservices'
            * S3 Signature Version              : 'None'
            * STS Session Duration in seconds   : '14400'

[venth]

Thanks you. Released with version 1.12.3